Stressing of post-tensioning cables.

Parking is a premium amenity for those living in urban areas. Located in the Crystal City neighborhood of Arlington, Va., the Buchanan House offers residents of the 200-unit luxury apartment complex nearly 600 parking spaces in a below-grade, three-story parking garage. In addition to the luxury residential units, the facility features restaurants and other retail shops on the street level, which results in additional parking needs. The consulting engineer for the project recognized that the post-tension concrete structure had begun to experience degradation from water infiltration. Additionally, the use of de-icing salts had caused corrosion of the existing buttonhead post-tensioning system.

A buttonhead post-tensioning systems consists of ¼-inch wires that are assembled typically in bundles of four to 12 wires per tendon.  During original installation, these bundled wires are fed through a stressing block or anchor.  A "button" is then pressed on the end of the wire and this acts as an anchor for the wire during stressing. The stressing block is comprised of two plates; each plate has holes that correspond with the number of wires to be stressed. The wires are pushed through the holes in the plates. On the opposite side from which they are running, buttons are then pressed on the wires. To stress the wires, a ram is dropped between the plates and they are forced apart. Then, two additional plates are installed between the stressing plates to maintain the required elongation. After this part of the installation, the jack is removed. One of the challenges with this system is that all the wires have to be the exact same length for the system to work properly.

Full-depth hole for new drain  installation.

Known for their excellent reputation with concrete repair projects, the repair contractor was contacted by the consulting engineer to work on the project. The repair contractor worked with the post-tension system supplier to select a repair strategy that would require less labor and would be more cost-effective than a complete system replacement. With the goal of restoring the parking garage's structure and improving its appearance, the repair contractor began the project by removing the damaged and delaminated concrete. This was necessary to thoroughly assess the condition of the existing buttonhead post-tensioning system components.

The parking structure's slab itself was reinforced by a two-way buttonhead post-tension system.  In most cases, repairs were required for most of the 4 to 12 wires contained in the tendon bundles. Some tendons had numerous breaks along their length that required multiple repairs. Also, there was the unknown of potential damage to the tendons in other locations not yet exposed. The engineer had to determine the forces to stress the tendons, based on these situations that would work through the project.  One discovery during the project was a tower crane opening on two floors, which resulted in tendons being "pushed" out of profile in order to wrap around the tower crane opening. Additionally, some of the tendons were anchored on the perimeter of the tower crane opening and in these areas, corrosion had occurred.  To repair this area, the concrete infill where the tower crane opening had been located was completely removed.  Reinforcing was added and the correct profile was then established for the tendons.

Overall, the repair contractor repaired 387 buttonhead post-tensioning tendons. To accomplish this, the repair contractor utilized a proprietary system from the post-tension system supplier that allowed the team to incorporate new monostrand post-tensioning materials without having to replace sections with a completely new buttonhead system.

This was a unique project because the repair contractor used monostrand and buttonhead post-tensioning together to regain the forces in the structure. By using the monostrand post-tensioning repair system, the team was able to reduce costs by having less labor on the project and decreasing the amount of concrete demolition needed to make the repairs. In addition to the post-tensioning repair, the repair contractor also performed partial and full depth concrete repair, as well as soffit, wall, column and beam repairs.

In addition, a corrosion-inhibiting coating was applied to 300,000 square feet of the parking garage in two passes, and a urethane coating system was installed to 211,000 square feet. These coatings will increase the service life of the structure. During application, full-face protection masks were used by the crew because the worked occurred in an enclosed area and the high PH level in the material could cause eye injury. 

The repair contractor also installed new handrails on the ramps to meet Americans with Disabilities Act (ADA) requirements. To improve the appearance of the garage, the entire structure was painted, including the painting of new parking stripes and directional indicators.

Installing coating detail on column.

The project was completed in six dual phases, meaning that phases one through six were completed sequentially and consisted of demo, repair and new concrete pours to fill the repaired areas of the slab.  At the end of phase six, the repair contractor went back to the phase one section and applied the corrosion-inhibiting coating and urethane coating system. Then, the repair contractor continued with the coating applications for the remaining phases. This allowed for the project schedule to move more efficiently and allowed for enough time for the concrete to cure prior to the coating application, painting and other finishing segments of the project. 

One of the major challenges for this project was dust control. The space was completely enclosed, and all exhaust fans were located on one side of the garage. To combat this, the repair contractor created tunnels to draft the dust out of the work area. Safety was paramount on this project.

Additionally, because the retail shops were still open and the residential portion of the building was still occupied, the garage remained open during repairs.  As a result, the repair contractor had to ensure proper signage was available to direct both vehicular and pedestrian traffic. Only one of the two exits was equipped with a pay station for those who do not have keycard access. During the repairs, a new pay site was established to ensure patrons to the retail shops who did not have a keycard could exit the garage. Phasing and scheduling was also crucial on this project, as several other trades were involved on the project.  Coordination between the different trades was critical to ensure that everyone maintained their schedules so that the there was no downtime between any of the six dual phases of the project.

Work began on this project in December 2006 and was completed in May 2008. Both the owner and visitors to the garage have commented on the visual appeal of the garage and are pleased with the outcome.  The project demonstrates how new technology can be utilized to repair existing specialty reinforcing systems to provide for a long-term and more cost-effective solution for the structure's owner.